It is known that during dielectric-fiber drawing, plant equipment must be adjusted so as to obtain a uniform section fiber, and in case of circular section, a constant diameter fiber. Manually or automatically effected corrections of possible variations in physical fiber dimensions require a continuous and up-dated knowledge of the values of the dimensions. Yet, the measurements must not interfere with the fabrication process. Hence, contact measurements are to be avoided. In this case, as is known, remote measurements can be made, using optical or electric methods.
An electric measuring apparatus is described in Italian Patent Application No. 67936-A/81, filed on July 6, 1981, in the name of the Applicant and published on Feb. 12, 1983 as European Patent Application, with the publication No. 69332. In this apparatus a fiber, while being drawn, is caused to pass between the plates of a parallel-plate capacitor, and detection is effected of capacitance variations due to the changes in the effective dielectric constant of the space between the plates, caused by the introduction of the fiber into said space and by diameter variations along the fiber which, together with air or another gas contained in the space between the two plates, forms the capacitor dielectric.
The magnitude of the diameter or of diameter variations is derived from the capacitance variations. A number of disadvantages are encountered when using such apparatus in a manufacturing plant, owing to vibration of the fibres while passing between the capacitor plates. Such vibrations cause variations in the capacitance of the measuring capacitor and hence may be erroneously read as diameter variations.
This phenomenon is due to a lower electric field intensity near the plate edges, where the lines of force bend and are more widely separated. Thus, electric capacitance depends also on the position of the dielectric fibre between the plates and decreases when the dielectric-fiber axis, in its translatory movement orthogonal to the electric field, moves closer to the edges. Thus the fiber may appear to be smaller than it actually is.
This disadvantage cannot be overcome by increasing the distance between the plate edges and the fiber under test, as this entails plate expansion and hence an increase in the capacitance C of the capacitor. This capacitance increase is detrimental to sensitivity and, hence, to a precise measurement. In fact the relative capacitance variation (.DELTA.C)/C, due to the introduction of the optical fiber with radius a into the space between the plates is given by the formula EQU (.DELTA.C)/C=(.pi.a.sup.2)/S.(.epsilon..sub.1 -.epsilon..sub.0)/(.epsilon..sub.1 +.epsilon..sub.0) (1)
where S is the surface of the section, orthogonal to the capacitor plates, assumed to be rectangular, and to the fibre axis, .epsilon..sub.1 is the dielectric constant of the material of the fibre, .epsilon..sub.0 is the dielectric constant of the medium (air or another gas) which surrounds the fibre in the space between the two capacitor plates, and C is the capacitance of the capacitor when the dielectric is wholly a medium of dielectric constant .epsilon..sub.0.
From formula (1) it will be apparent that such relative variation is inversely proportional to S, which therefore is to be kept small. For example, in an apparatus using a capacitor with a capacity of 1 pF, there is a variation of about 0.003 pF after the introduction of a fibre having a diameter of about 125 .mu.m and dielectric constant .epsilon..sub.1 equal to about 4, while the measuring instrument must possess enough sensitivity to detect diameter variations of 1/1000 and hence capacity variation of 6.multidot.10.sup.-6 pF.
It is to be noted that the vibration component which is in the plane parallel to the plates is usually more detrimental than that in the perpendicular plane; however, it is advisable to minimize the effects due to both components.
In addition to capacitance variations due to fiber vibrations, there are also variations due to thermal expansion of plates and of other structures forming the capacitor and variations due to measuring instruments, for which suitable compensation should be provided.